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\begin{equation*}\hat{\textbf{P}}_{k|k-1} = \sum_{i=1}^N W_{c}^{i}\ [\chi_{k|k-1}^{i} - \hat{\textbf{x}}_{k|k-1}] [\chi_{k|k-1}^{i} - \hat{\textbf{x}}_{k|k-1}]^{T} \end{equation*}
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